Investigation Of Reliability For The Copper Pillar Bump Interconnection In The Flip Chip Packaging Under Thermal And Current Stressing

The copper pillar bump in flip chip is an advanced integrated circuit(IC)packaging technology,which is applied to the interconnection between silicon chip and substrate.It has been widely used in the flied of military,health,and mobile electronic industries because of the advantages of fine pitch,micro-scale,ultrahigh input/output(I/O)density.However,with the development of IC manufacturing techniques,the bump pitch continues to shrink and load current density increases quickly.The joule heat and electro-migration(EM)effect in packaging has more significant influence on solder joints.In this paper,the temperature,current density and stress distributions in copper pillar bumps are simulated by using the finite element analysis software ANSYS.The infrared thermography is used to verify the simulation of temperature distribution.The results show the growth relationship of temperature and stress distributions as current density increased.And the high stress areas and critical current density which could lead to thermal migration of solder are determined.The Cu/Sn/Cu interconnection structure has been taken as research subject to analyze Cu migration behaviors in lead-free solder.A theory model is established to investigate the polarity effect of EM on kinetics of intermetallic compounds(IMC)growth.The results show that,while under current stressing,EM Cu flux in Sn solder accelerates anode's IMC growth and motivates transition of its thickness growth rule.The growth of cathode's IMC is influenced by the dynamic equilibrium of Cu dissolution process and Cu diffusion process.Note that,the initial thickness of IMC affects the IMC growth rule on cathode side.Combined simulation with reliability testing,failure mechanism for copper pillar bump during current stressing is studied.The results show that thermal mechanical stress is an important reason leading to the failure for copper pillar interconnection.During isothermal aging,Ni layer is preventing from the alloying reaction of copper pillar,which caused the enhancement of Cu pad consumption.While under current stressing,the growth of IMC layer is facilitated.And interfacial reaction in solder bump present obvious polarity effect.Finally,based on the Black's equation,the mean-time-to-failure of reliability test was analyzed to determine the exponent for current density and activation energy for copper pillar interconnection.The EM reliability model of copper pillar bump interconnection is established.Combined simulation with test,the temperature rise caused by Joule heat effect under different current density is calculated.The influence of Joule heat effect and current crowding effect is considered to adjust temperature parameter and current density for the purpose of modifying the reliability model.